1. Field of the Invention
The present invention relates to a circuit that pulls down the power supply line (power rails) in an electronic system so that the power supply line goes to a low state and stays there when the electronic system is turned off. More specifically, the present invention relates to a circuit that pulls the power supply line low in a computer or other electronic system when the computer is turned off so that there is no residual voltage or back feed voltage on the power supply line when the system is turned off.
2. Description of Related Art
In the past when a power supply to a computer is turned off, the five volt power line was allowed to float at whatever voltage it naturally may have gone to. Most of the time the five volt power line would go to ground, unless an active peripheral device presents a bias to the power line through parasitic or "sneak" path. The active peripheral may thus cause the power line to be driven to a non-zero value. The non-zero voltage on the power line, which provides power to the various devices in a system, may cause system initialization problems when the system is turned on. Examples of system initialization problems include unrecognizable or inoperable system functions. A real life example could be a peripheral system, such as a mouse driver or networked interface cards, may not initialize correctly.
To remedy the residual voltage on the power line problem a passive technique has been used. FIG. 1 depicts an example of the passive technique. The passive technique requires a resistance, Rb, to be placed between power and ground. The passive technique corrects most of the problems of residual voltage on the power line when the system is turned off depending on the resistance value and the amount of current available from the biasing source. There are also drawbacks to the passive technique.
The drawbacks of the passive technique for pulling the power line low when the system is off include a decrease in the efficiency of the total system because when the system is on there is a resistor connected between the power line and ground consuming energy and doing basically nothing As a result, the power consumption of the system increases and the efficient use of energy decreases This is a significant drawback when complying with "Energy Star" requirements.
Thus, there is a need for a circuit that pulls a power supply's lines low or to ground when a computer or an electronic system is turned off, but dissipates no or an insignificant amount of power when the system is turned on. The present invention will accomplish both of these objectives